Technology update

Oct 3, 2013

Quantum communication moves forward

Researchers at the University of California at Santa Barbara have made a nanomechanical device that can convert electrical quantum signals into optical quantum ones. Although the prototype device currently operates in the classical regime, it could be adapted to work as a quantum transducer in the future.

“It has recently become possible to control the mechanical motion of on-chip nanostructures at the quantum level. This opens the door to a completely new area of research on ‘hybrid quantum systems’, from which new quantum-based technologies might see the light of day,” said team leader Andrew Cleland. “Being able to link electronic quantum systems to optical communication channels is of paramount importance because today’s leading quantum information processing technology is based on electronic (superconducting) quantum bits. However, to communicate quantum information over any distance, optical channels must be employed.”

The new device could ultimately serve as a critical element in such a quantum communications network, he added.

The researchers made their transducer using lithographic processing on standard silicon wafers. This technique can easily be scaled up to produce the devices in large quantities. All of the functional elements, including piezo-electromechanical and photonic circuits, are integrated into a thin (330 nm) suspended beam made of the piezoelectric material aluminium nitride.

“We can think of the device as a piezoelectric photonic crystal in which an engineered phonon mode – a vibration of the crystal lattice – boosts the interaction between electrical microwave and optical photons,” explained Cleland. “It is in fact an electro-optical modulator for the quantum regime.”

The team exploited the piezoelectric effect in aluminium nitride to convert an ingoing electrical microwave signal to a GHz frequency vibration of the suspended nanobeam. As well as acting as a piezo-element, this beam is patterned into both a photonic and phonic crystal. In this so-called optomechanical crystal (a concept first put forward by Oskar Painter’s group at Caltech), light and matter strongly interact. The structure thus efficiently converts mechanical vibrations onto modulation sidebands of optical probe light, says Cleland, and the incoming light is routed from a fibre-optic coupler to an on-chip optical waveguide that couples to the nanobeam.

Compatible with superconducting circuits and standard fibre optics

“The main advantage of our approach is that the device layout, all fabrication steps and the operating frequency range (of between 4 and 6 GHz) are compatible with the superconducting quantum circuits actively being developed in several other labs around the world – for example, at UCSB, Yale, IBM and ETH Zurich, among others,” Cleland told nanotechweb.org. “Our transducer will integrate well with these superconducting circuits.”

The device is also optimized for operating in the all-important 1550 nm telecommunications optical wavelength and is therefore compatible with standard fibre optics technology, he adds. “In fact, our actual device chip is directly connected to optical fibres and we did our experiments using ordinary telecoms equipment.”

The proof-of-principle device proves that making a nanomechanical transducer is indeed feasible, he says. “We will now be looking at improving the device design as well the coupling between the electrical, mechanical and optical modes in it. At the same time, we need to work on integrating these next-generation transducers into real-world superconducting quantum circuits.”